POLYGYRA. 



263 



Fig 157. 



Jaw of 

 S. ventrosula. 



Jaw high, arcuate, ends scarcely attenuated, blunt, cutting edge without 

 median projection ; anterior surface with numerous stout, separated ribs, den- 

 ticulating either margin. I have counted 8 ribs in P. venlrosula; 14 in pus- 

 tula; 10 in auriculata; 12 in Postelliana ; 12 in Carpcnteriana ; 

 10 in pusluloides ; 12 in avara ; over 14 in cereolus ; 10 in espi- 

 loca; 13 in uvulifera ; 10 in Texasiana and Iriodontoides : 12 in 

 Troostiana; 11 mleporina; 15 in Mooreana ; 20 in fastigans ; 

 7 in septemvolva; 10 in Febigeri; in Hazardi and auriformis 

 they are also numerous. I have had no opportunity of examin- 

 ing the jaw in the other species found within our limits Hindsi, tholus, 

 hippocrepis, oppilala, Dorfeuilliana, Ariadnce, 



By the character of its jaw, Polygyra can be compared only io Triodopsis 

 and Mesodon among the other North American subgenera of Helix. No foreign 

 species has yet been examined. 



Fig. 158 shows the general arrangement of the teeth upon the lingual mem- 

 brane, the characters of the individual teeth being better shown in my PI. VI. 



Fig. 158. 



Lingual dentition of P. auriformis? (Leidy). 



The teeth do not differ from what I have described under Stenotrema (see 

 below). As in all the subgenera, there is considerable difference in the length of 

 the base of attachment on the central and lateral teeth in the several species. 



I find considerable difference between the various species in the manner in 

 which the lateral teeth pass into the marginals. In auriformis, Postelliana, 

 espiloca, and Hazardi, the change is made simply by the greater development 

 of the inner cutting point, not by its bifurcation (see PI. VI. Fig. N). In these 

 species it is only the extreme outer marginals that have their inner cutting 

 point bifid; in auriformis a very few extreme marginals have a bifid cutting 

 point. This species has very long inner cutting points to its marginal teeth. 

 In the other species examined by me the first marginals have their inner 'cut- 

 ting po'nt bifid, the transition from laterals to marginals being thus very dis- 

 tinctly marked (see PI. VI. Fig. K). With these exceptions, the dentition of 

 our species of Polygyra is very like that of Stenotrema (q. v.). 



The dentition of no foreign species is known with which to compare our 

 species. 



Polygyra auriculata, Say. 



Shell rimately perforated, flattened above, inflated below, with rib-like striae, 

 reddish horn-color or brownish ; whorls 5|, narrow, the last deflected at the 

 aperture, disjoined, constricted and scrobiculated below ; umbilicus level, show- 



